This invention relates to the construction of an ultrasonic motor.
A conventional ultrasonic motor consists of a vibrator provided on one surface thereof with a piezoelectric element bonded thereto, and on the other surface thereof with a comb-like projections arranged alternately in the circumferential direction thereof, and a movable member engaged under pressure with the upper surface of the comb-like projections of the vibrator. In such an ultrasonic motor, an electric signal is applied to the piezoelectric element to generate a travelling wave in the vibrator and rotate the movable member by utilizing the lateral amplitude of this wave, and the comb-like projections play a role of extracting a large amplitude without exerting any influence upon the travelling wave in the vibrator.
In such an ultrasonic motor, the efficiency is increased by the comb-like projections provided on the vibrator but it varies due to the positional relation between the comb-like projections and the electrode patterns on the piezoelectric element. Referring to, for example, FIG. 4, a vibrator 1 is provided with a comb-like projections la which are hatched. A piezoelectric element 2 is provided on the rear surface thereof with 12 equally shaped electrode patterns 2a-2d spaced regularly in the circumferential direction thereof. The regions of two adjacent electrode patterns 2a, 2b are polarized positively, and those of two adjacent electrode patterns 2c, 2d negatively. Out of the lead wires 5 attached to these electrode patterns 2a-2d, the lead wires attached to the alternate electrode patterns 2a, 2c are bundled 5a, while the lead wires attached to the other alternate electrode patterns 2b, 2d are bundled 5b, signals which have a 90.degree. time phase difference being applied to the lead wires 5a, 5b. Consequently, a travelling wave occurs in the vibrator 1. Although this wave is a travelling wave, it is a combination of the standing waves in the lead wire group 5a and those in the lead wire group 5b. The details of these standing waves are illustrated in FIG. 5, in which the standing waves occurring in the electrode patterns 2a, 2c; 2b, 2d are designated by the letters A and B, respectively. The number of the projections on the vibrator 1 is 24, and the vibrator 1 is bonded to the piezoelectric element 2 so that the center lines of the projections 1e and those of the electrode patterns 2a-2d are aligned. In this case, the peaks C (12 positions) of the standing waves and the center lines of the projections le are aligned. Accordingly, the bending rigidity of this vibrator 1 is high, and the flexure thereof is small. On the other hand, in a vibrator bonded to a piezoelectric element so that the center lines of the recesses 1f in the comb-like projections thereof and those of the electrode patterns 2a-2d are aligned as shown in FIGS. 6 and 7, the peaks C (12 positions) of the standing waves are aligned with the center lines of the recesses 1f. Accordingly, the bending rigidity of this vibrator becomes low, so that this vibrator necessarily bends largely. As is clear from the above, a conventional ultrasonic motor has the problem that the efficiency thereof varies greatly due to the scatter of the portions of the vibrator 1 and piezoelectric element 2 at which these two parts are bonded together.